1. Field of the Invention
This invention relates to a non-destructive tap for an optical fiber, and especially to a tap used to provide a node in a ring topology data processing local area network.
2. Introduction to the Invention
In the past few years, as large scale integration of semiconductor circuitry has made microprocessors and microcomputers available, there has arisen an increased interest in methods of efficient interconnection into what are usually referred to as local area networks. The aim of the local area network is the connection of data processing nodes (terminals, computers, etc.) in such a manner that they may communicate with each other, sharing resources and exchanging information. Such networks are now commercially available from such manufacturers as Xerox Corporation (Ethernet.TM.) and Wang Laboratories (Wangnet.TM.).
Two general classes of physical link topologies are employed for these networks: bus and ring. The bus topology provides a bus which operates as a bidirectional transmission facility to which all the nodes are attached. Signals from one node enter the bus and propagate down it in both directions to its ends, where they are terminated. Such a topology provides a passive attachment mechanism: nodes may be added to or deleted from the network without disruption. In the ring topology, however, the nodes are connected by unidirectional transmission facilities to form a structure with a single closed path. Signals placed on the ring pass from node to node and are regenerated as they pass through each node. The attachment node has been referred to as active attachment, enabling all attached nodes to maintain synchronization. A disadvantage of the ring topology is, however, that the addition or deletion of nodes disrupts the network.
Approximately contemporaneously with the increase in networks has been interest in optical fibers as a data communications medium. Optical fibers offer substantial advantages over electrical conductors, particularly in bandwidth (enabling higher data transmission rates) and in immunity to electrical noise (requiring less shielding). Two principle disadvantages are fragility and difficulty of connection. Adequate techniques are now available for jacketing and strengthening optical fiber cables so that they can be used virtually as easily as conventional communications cable, and certainly as easily as high-bandwidth coaxial cable, and techniques for connection continue to improve.
It is when an optical fiber is proposed to be used as the communications medium in a ring network that a real disadvantage of optical fibers appears. When a node is removed from a ring network, the break in the ring occupied by the detector-data processor-transmitter of that node must be filled to establish connection between the nodes on either side of the removed node. For electrical circuitry, this is comparatively easy: a switch can be used to break the line and connect it to the node. The deterioration of signal quality when the node is switched out of the ring is minimal. However, when an optical fiber is used, connections cannot be as easily made and broken.
What is desired is an optical fiber tap which, when it is not activated, causes essentially no loss in the optical fiber and which, when activated, switches essentially the entire optical signal into/out of the node.
In U.S. patent applications Ser. Nos. 258,079 (filed Apr. 27, 1982, now abandoned), 370,321 (filed Apr. 21, 1982, now abandoned) and 437,053 (Filed Oct. 27, 1982, now abandoned), all of which are assigned to the assignee of the present application and the entire disclosures of which are incorporated herein by reference, there is described an apparatus for splicing two optical fibers. The apparatus incorporates means for coupling light into and out of the fibers to be spliced to aid alignment.